A device for locking and unlocking a door without a key is described in "Keyless Entry System with Radio Card Transponder," Motoki Hirano et al., IEEE Transactions on Industrial Electronics, Vol. 35, 1988, pages 208 through 216. This conventional device is used to unlock a locked door of a motor vehicle as soon as a user carrying a transponder enters the interrogation field of a transmitting unit located in the door region, triggering interrogation. The door is unlocked when the transmitting unit detects the correct transponder code, using an evaluator.
Although it is not difficult for the user to lock and unlock the door using such a device, situations can arise in which the mode of operation of this device is disadvantageous, making it less attractive to users. For example, it is possible to start a vehicle if an unauthorized user gains access to the car's interior.